This invention relates to suppression of vibration during seek operations in a disc drive, and particularly to a filter for filtering bang-bang signals representing command profiles to suppress vibration modes.
Disc drives employ discs having concentric data tracks and a head movable radially across the disc. The head is mounted on an actuator arm that is moved by an actuator, such as a voice coil motor, driven by a command signal. The command signal is selected by a non-linear look-up table containing representations of position velocity profiles to provide signals to the actuator. Ideally, the signal to the actuator is a full on, full off signal known in the trade as a bang-bang signal. The bang-bang signal operates the actuator to accelerate the movement of the actuator arm radially across the disc, followed by a deceleration cycle to bring the head to the desired track. For long seeks, a period of zero acceleration (constant velocity) may appear between the acceleration and deceleration cycles. Track counting is employed to count tracks during the seek operation to enable the controller to control the acceleration and deceleration cycles to bring the head to the destination track in the shortest amount of time. One problem with the seek operation is that mechanical resonance of the disc drive, including in the actuator arm itself, generates vibration, and therefore noise, during the seek operation. Additionally, mechanical impulses, applied to the actuator arm, also generate vibration and acoustic noise. Vibration is a particularly troublesome problem in seek systems employing bang-bang signals due to impulses created by sudden changes in acceleration in the arm movement. Vibration in the actuator arm and disc drive adversely affects the ability of the actuator arm to center the head on the desired track on the disc, thereby increasing the access time to the desired track. Noisy seeking requires additional time for damping out residual vibration.
Previously, several techniques had been employed to overcome the difficulties associated with vibration generated during track seeks. One technique is to simply permit the vibration to dampen, but this technique necessarily adds to the seek time.
Another technique employs an electrical model of the mechanical system. The drive signal operates the model to produce a series of signals representative of vibration of the system. These signals are monitored for peaks exceeding a predetermined threshold, in which case the servo is operated to delay the command to permit the vibration to dampen. An example of this system is found in Rickert, U.S. Pat. No. 4,477,755.
Another technique is to add a signal to the bang-bang signal to create a sinusoidal profile to the acceleration profile generated by the bang-bang signal. The added signal is representative of the mechanical noise and vibration of the disc drive. The generated acceleration profile represents a reasonably smooth motion of the actuator arm from the start track to the destination track. Examples of adding correction signals may be found in Andrews, U.S. Pat. No. 5,291,110, and in Miu, xe2x80x9cMinimum Power and Minimum Jerk Position Controls and its Application in Computer Disk Drivesxe2x80x9d, IEEE Transactions on Magnetics, Vol. 27, No. 6, Pages 4471-4475 (November 1991). While the approach of adding correction signals does not introduce objectionable delay to seek operations, these techniques act to cancel the effects of natural resonance or vibration of the system, rather than eliminate the vibration altogether.
In one form of the invention, a controller provides a series of bang-bang commands to operate the voice coil motor, or actuator, for an actuator arm of a disc drive containing at least one disc. A filter is responsive to selected ones of the bang-bang commands to provide an operating signal to the actuator representing the selected bang-bang command with damped amplitudes of dominant vibration mode frequencies of the selected bang-bang command.
A preferred form of the apparatus includes a finite impulse response filter having a discrete transfer function of C(1+al1zxe2x88x92l+al2zxe2x88x92lxc2x72+ . . . +alrzxe2x88x92lxc2x7r), where C is a constant, al1, al2, . . . alr air represent impulse amplitudes at spaced positions within a sample period and zxe2x88x92lxc2x71, zxe2x88x92lxc2x72, . . . zxe2x88x92lxc2x7r represent delay components at the respective spaced positions.
In another preferred form of the apparatus the finite impulse response filter has a discrete sampling period, T, that is an integer multiple, l, of the controller sampling period, ts, with l being as small as practical to maintain an all-positive amplitude solution for the filter with a short time duration. One example for integer l is 2.
Optionally, the apparatus includes a second filter, such as a low pass or band pass filter, for removing representations of frequencies in the selected bang-bang command above a designated frequency.
In another form of the invention, a method is provided for suppressing seek-induced vibration in a disc drive during seek operation. The method comprises providing a finite impulse response filter having a discrete transfer function of C(1+al1zxe2x88x92l+al2zxe2x88x92lxc2x72+ . . . +alrzxe2x88x92lxc2x7r), where C is a constant, al1, al2, . . . alr represent impulse amplitudes at spaced positions within a sample period and zxe2x88x92lxc2x71, zxe2x88x92lxc2x72, . . . zxe2x88x92lxc2x7r represent delay components at the respective spaced positions. A bang-bang command for performing the seek operation is filtered using the finite impulse response filter to derive an operating signal, and the head is positioned based on the operating signal.
In a preferred form of the method, the finite impulse response filter damps amplitudes of selected frequencies by pole-zero cancellation. The discrete transfer function is derived based on a conjugate pair of poles in the s-domain for each of a plurality of vibration frequencies of             s      i        =                            -                      ζ            i                          ⁢                  ω          ni                    +                        jω          ni                ⁢                              1            -                          ζ              i              2                                      ⁢                  xe2x80x83                ⁢        and                        s      i      *        =                            -                      ζ            i                          ⁢                  ω          ni                    -                        jω                      n            ⁢            i                          ⁢                              1            -                          ζ              i              2                                          
where xcex6i is the damping ratio and xcfx89ni is the natural frequency for the i-th mode.
Optionally, the method also comprises removing frequencies in the selected bang-bang command above a designated frequency.
These and various other features as well as advantages which characterize the present invention will be apparent upon review of the following detailed description and the associated drawings.